Yes, sometimes finding other people can be problematic, but that's not exactly a game issue, it's a person issue. There are plenty of us who work full time (I work 12 hour shifts, (3/12's) and still make the effort.
I promise, playing against people is worth the effort.
Not even referring to GC3 specifically- AI is generally garbage, in nearly every game. That's why AI difficulties are always setup to provide massive bonuses to the AI. It's the only way to make them even remotely capable of playing against a person.
Complain about anything else you want to about a game- that's fair, but complaining the garbage AI isn't quite up to par with some other garbage AI you've played is just baffling to me. Even more so when you consider the amount of development time that will be shoveled into a garbage chute to make said garbage AI slightly less garbage. There's obviously some glaring issues sometimes, but most of the time even a thousand hours of work will never make the AI more than absolute garbage.
Maybe I have a unique perspective on it since I have friends who've worked on RTS AI, and have listened to them talk about how borderline improbable it is to make an AI thats even semi decent :/
Ultimately, you can't just dismiss "play with people" because it's inconvenient for you. It's a perfectly legitimate option that many people do every day. You should make the effort to participate, which is why I even suggested it to begin with. There's far too many good things about this game (and many others of similar genre) to pass up purely because of the AI.